German Patent Application No. DE 199 03 439 describes a method and a device for operating an internal combustion engine in whose exhaust area an SCR (selective catalytic reduction) catalytic converter is provided which uses a reagent to reduce nitrogen oxides in the exhaust gas of an internal combustion engine to nitrogen. The reagent is preferably metered as a function of performance characteristics of the internal combustion engine such as the speed and the quantity of fuel injected. In addition, fuel is preferably metered based on operating parameters of the exhaust gas, such as the exhaust gas temperature or the operating temperature of the SCR catalytic converter.
For example, the reducing agent ammonia, which may be obtained from a urea-water solution, is used as the reagent. The reagent or precursors of the reagent must be metered carefully. If the amount metered is too low, this results in nitrogen oxides no longer being completely reduced in the SCR catalytic converter. If the metering is too high, this results in leakage of reagent, which may result in unnecessarily high reagent consumption as well as an unpleasant odor problem, depending on the nature of the reagent. In addition, another factor to be taken into account is that ammonia is toxic.
German Patent Application No. DE 197 39 848 describes a procedure using which crude NOx emissions of an internal combustion engine may be calculated at least approximately from known performance characteristics of the engine. The starting point is an engine characteristics map, which is based on the speed and torque of the engine. In addition, corrections may also be provided, e.g., as a function of the lambda value.
German Patent Application No. DE 10 2004 031 624 (not a prior publication) describes a method for operating an SCR catalytic converter used for purifying the exhaust gas of an internal combustion engine; with this method, control or regulation of the reagent filling level in the SCR catalytic converter is provided at a predefined setpoint storage volume. The targeted specification of the setpoint storage volume ensures that an adequate quantity of reagent for the most thorough possible elimination of crude NOx emissions produced by the internal combustion engine is available in non-steady states of the engine while avoiding reagent leakage.
The reagent filling level of the SCR catalytic converter is determined on the basis of a catalytic converter model that takes into account the NOx mass flow entering the SCR catalytic converter, the NOx mass flow leaving the SCR catalytic converter, the catalytic converter temperature and, if necessary, the reagent leakage. The maximum possible reagent filling level of the SCR catalytic converter depends in particular on the operating temperature of the SCR catalytic converter. The maximum possible reagent filling level is highest at low operating temperatures and drops toward lower values with increasing operating temperatures. The efficiency of the SCR catalytic converter depends on the catalytic activity, which is also low at low operating temperatures, passing through a maximum with an increase in operating temperature and then dropping again with a further increase in operating temperature.
German Patent No. DE 199 60 731 describes an NOx sensor which detects the NOx concentration in an exhaust gas stream of an internal combustion engine. The circuit system provided for operating the NOx sensor permits an adjustment of the potentials with a very small offset relative to one another.
German Patent No. DE 199 62 912 also describes an NOx sensor for detecting the NOx concentration in an exhaust gas stream. A circuit system changes the voltages supplied as a function of the currents flowing in the electrode leads or the currents flowing between the electrodes in such a way that the voltages applied to the electrodes inside the sensor have the predefined setpoints.
The NOx sensors described here have multiple chambers interconnected through the diffusion barriers. The O2 concentration is reduced to a predetermined value in the first chamber by a first electrolytic oxygen pumping cell. Conditions in the first chamber such as temperature, catalytic effect of the electrode material and pumping voltage are selected to prevent decomposition of the NOx into N2 and O2 . In the second chamber, O2 is also pumped out. In the second chamber, O2 is again pumped out at a second pump electrode down to a very low O2 concentration. This promotes an NOx reduction reaction. Electrochemical reduction then takes place at a third pumping electrode. The intensity of the pumping current across the third electrode in the second chamber may thus be used as a measure of the NOx concentration in the exhaust gas. A third chamber is connected to ambient air and contains an air reference electrode. Known amperometric dual-chamber NOx sensors have a cross-sensitivity to ammonia (NH3) due to the measurement principle. Ammonia present in the exhaust gas, as an example of a reagent, results in distortion of the sensor signal due to the reaction 4NH3+5O2→4NO+6H2O.
An object of the present invention is to provide a method for operating an internal combustion engine in whose exhaust area at least one catalytic converter and an NOx sensor are provided and a device for carrying out the method using which the least possible reagent leakage of a reagent required in the catalytic converter occurs.